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Answer to 3
Cameras with zoom function should have a feature called backfocus set at the factory. Cameras with fixed(non-removable)zoom lenses tend to not have this as a user adjustment.
Those that allow the changing of zoom lenses should have a backfocus adjustment(a feature that is on the more high end cameras). If properly set, you should have your object in focus throughout the zoom. To test this properly you want to zoom in first (for the greatest detail in your image) and set your focus. As long as the distance between you and your subject doesn’t change, it should remain in focus through the zoom.
This backfocus feature (a small focus lens between camera and yourzoom lens and usually has some locking mechanism to prevent it from moving after it is set, see your cameras operators manual) allows for compensating for the different distances from the zoom lenses to the camera’s CCD chip.
An “off the shoulder” calibration is to zoom in on something a distance away (building with lots of windows for example) adjust focus so the details are as clear as possible, zoom out to widest field. If backfocus is set properly the building (or what ever object you are using) should remain in focus. If not adjust your backfocus lens (not normal focus, see your cameras operators manual) and repeat these steps until the focus is unchanged throughout the zoom.